1. Field of the Invention
The present invention relates to an MR imaging method, a phase shift measuring method and an MR (Magnetic Resonance) imaging system. More specifically, the present invention is concerned with an MR imaging method using a pulse sequence, which is capable of preventing the influence of variations in residual magnetization due to a change in gradient or gradient magnetic field pulse, a phase shift measuring method for measuring phase shifts in subsequent echoes due to the influence of eddy currents and residual magnetization caused by each preceding phase encoding pulse or the like in the pulse sequence, and an MR imaging system for executing these methods.
2. Description of the Related Art
The following prior arts have been disclosed in Japanese Patent Application Laid-Open No. Hei 10-75940.
(1) A phase shift measuring method for executing a pre-scan sequence for transmitting an excitation pulse, transmitting an inversion pulse, applying a phase encoding pulse to a phase axis, applying a read pulse to a read axis, applying a rewind pulse to the phase axis, continuously transmitting an inversion pulse, from echoes while a read pulse is being applied to the phase axis, and measuring a phase shift in each subsequent echo due to the influence of eddy currents and residual magnetization caused by each phase encoding pulse or the like, based on phase data obtained by converting the collected data into one-dimensional Fourier form.
(2) An MR imaging method wherein in a pulse sequence using a high-speed spin echo process for transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while a read pulse is being applied to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, and collecting data of a plurality of echoes by one excitation, a compensating pulse for compensating for the amount of the phase shift measured by the phase shift measuring method described in the paragraph (1) is built or integrated into a phase encoding pulse, added to one or both immediately before or after the phase encoding pulse, built in a rewind pulse or added to one or both immediately before or after the rewind pulse.
The prior arts have been based on the precondition that the amount of the phase shift measured by the phase shift measuring method described in the paragraph (1) and the amount of the phase shift produced when no compensating pulse is provided in the pulse sequence of the high-speed spin echo process described in the paragraph (2), are equal to each other.
In a permanent magnet type MR imaging system, however, the two are not necessarily made equal to each other due to a magnetic hysteresis characteristic of a magnetic shunt plate or the like. Thus, the precondition employed in the prior arts is not established. This will be explained with reference to FIGS. 1 through 3.
FIG. 1 is a pulse sequence diagram showing the conventional high-speed spin echo process.
In this FSE (Fast Spin Echo) sequence SQ, an excitation pulse R and a slice gradient or gradient ss are first applied. Next, a first inversion pulse P1 and a slice gradient ss are applied. Then a phase encoding pulse gy1i is applied to a phase axis. Next, an NMR signal is received from a first echo echo1 while a leas pulse gxw is being applied. Thereafter, a rewind pulse gy1ri equal in time integrated value and opposite in polarity to the phase encoding pulse gy1i is applied to the phase axis. Incidentally, i indicates repetitive numbers. i=1 through I (e.g., I=128).
Next, a second inversion pulse P2 and a slice gradient ss are applied, a phase encoding pulse gy2i is applied to the phase axis, and an NMR signal is received from a second echo echo2 while the read pulse gxw is being applied. Afterwards, a rewind pulse gy2r equal in time integrated value and opposite in polarity to the phase encoding pulse gy2i is applied to the phase axis.
A jth inversion pulse Pj and a slice gradient ss are applied subsequently in the same manner as described above. A phase encoding pulse gyji is applied to the phase axis. An NMR signal is received from a jth echo echoj while the read pulse gxw is being applied. Thereafter, the application of a rewind pulse gyjri identical in time integrated value and opposite in polarity to the phase encoding pulse gyji to the phase axis is repeated for j=3 to J (e.g., J=8).
Assuming that the residual magnetization prior to the application of a phase encoding pulse gy1l (i=1) is defined as m1, the residual magnetization traces histories a1 and a2 so as to reach m1 again by the application of the phase encoding pulse gy1l, as shown in FIG. 2 due to the magnetic hysteresis characteristic of the magnetic shunt plate or the like in the permanent magnet type MR imaging system. Further, the residual magnetization traces histories a3 and a4 so as to reach m2 by the application of a rewind pulse gy1rl.
Since the residual magnetization preceding the application of a phase encoding pulse gy2l becomes m2, the residual magnetization traces histories a5 and a6 so as to reach m3 by the application of the phase encoding pulse gy2l, as shown in FIG. 3. Further, the residual magnetization traces histories a7 and a8 so as to reach m2 again by the application of a rewind pulse gy2rl.
In the permanent magnet type MR imaging system as described above, the residual magnetization varies depending on the histories in which the gradient magnetic field pulses such as the phase encoding pulse and the rewind pulse are varied, due to the magnetic hysteresis characteristic of the magnetic shunt plate or the like.
However, since the pre-scan sequence takes such a form as obtained by cutting down ones up to the collection of the first echo in the FSE sequence, the gradient magnetic field pulses coincide in application history with each other and the residual magnetization makes consistence, with respect to the first echo. Therefore, the aforementioned precondition is established. However, since the gradient magnetic field pulses do not coincide in application history with each other in the second echo or later, the residual magnetization does not make consistence and hence the aforementioned precondition is no longer established.
Therefore, a problem arises in that in the second echo or later, a phase shift in subsequent echo due to the influence of the residual magnetization caused by each gradient magnetic field pulse cannot be sufficiently corrected.
Since a linear relationship is established between the area of each gradient magnetic field pulse and a generated phase error, the above-described problem is considered to be free from occurring in eddy currents.
A first object of the present invention is to provide an MR imaging method capable of preventing the influence of a variation in residual magnetization due to a change in gradient magnetic field pulse.
A second object of the present invention is to provide a phase shift measuring method capable of causing a phase shift in subsequent echo due to the influence of eddy currents and residual magnetization caused by a preceding phase encoding pulse or the like in an FSE pulse sequence to coincide with an actual FSE sequence to thereby measure the phase shift.
Further, a third object of the present invention is to provide an MR imaging system for executing the above-described methods.
In a first aspect, the present invention provides an MR imaging method using a high-speed spin echo process comprising the steps of transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while applying a read pulse to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, collecting data from a plurality of echoes by one excitation, and inserting a positive or negative reset pulse having amplitude equal to the maximum amplitude used in an arbitrary gradient axis or having greater amplitude into the arbitrary gradient axis before and behind the inversion pulse.
In the MR imaging method according to the first aspect, since the amplitude of the reset pulse is not smaller than that of each of other gradient magnetic field pulses, the residual magnetization produced after the application of the reset pulse is always kept constant regardless of the applied histories of other gradient magnetic field pulses. Further, since the reset pulse is inserted after the inversion pulse, i.e., immediately before the application of the phase encoding pulse, the residual magnetization produced before the application of the phase encoding pulse is always kept constant. It is thus possible to prevent the influence of a variation in residual magnetization due to a change in gradient magnetic field pulse.
Incidentally, the reset pulse does not influence phase encoding because it is also inserted before the inversion pulse.
In a second aspect, the present invention provides a phase shift measuring method comprising the steps of transmitting an excitation pulse, transmitting an inversion pulse, applying a phase encoding pulse to a phase axis, applying a read pulse to a read axis, applying a rewind pulse to the phase axis, continuously transmitting an inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, determining the amount of a phase shift due to the influence of the phase encoding pulse, based on phase data obtained by converting the data into one-dimensional Fourier form, and inserting a positive or negative reset pulse having amplitude equal to the maximum amplitude used in a phase axis or having greater amplitude into the phase axis before and behind the inversion pulse.
In the phase shift measuring method according to the second aspect, since the amplitude of the reset pulse is not smaller than that of each of other gradient magnetic field pulses, the residual magnetization produced after the application of the reset pulse is always kept constant regardless of the applied histories of other gradient magnetic field pulses. Further, since the reset pulse is inserted after the inversion pulse, i.e., immediately before the application of the phase encoding pulse, the residual magnetization produced before the application of the phase encoding pulse is always kept constant. Namely, the residual magnetization can be made coincident with the residual magnetization prior to the application of a phase encoding pulse in an FSE sequence. Thus, a phase shift in subsequent echo due to the influence of eddy currents and residual magnetization caused by the preceding phase encoding pulse or the like in the FSE sequence can be measured by matching with an actual FSE sequence.
Incidentally, the reset pulse does not influence phase encoding because it is also inserted before the inversion pulse.
In a third aspect, the present invention provides an MR imaging method according to the first aspect, wherein a compensating pulse for compensating for the amount of the phase shift measured by the phase shift measuring method according to the second aspect is built in each phase encoding pulse, added to one or both immediately before or after the phase encoding pulse, built in each rewind pulse, or added to one or both immediately before or after the rewind pulse.
In the MR imaging method according to the third aspect, since the compensation for the amount of the phase shift, which could be measured by matching with the actual FSE sequence, is performed, a phase shift in subsequent echo due to the influence of eddy currents and residual magnetization caused by each preceding phase encoding pulse or the like in an FSE pulse sequence can be corrected accurately.
In a fourth aspect, the present invention provides a phase shift measuring method comprising the steps of transmitting an excitation pulse, transmitting an inversion pulse, applying a read pulse to a read axis, continuously transmitting an inversion pulse, applying a dephaser pulse to a phase axis, collecting data from echoes while applying a read pulse to the phase axis, acquiring first phase data by converting the data into one-dimensional Fourier form, inserting a positive or negative reset pulse having amplitude equal to the maximum amplitude used in the phase axis or having greater amplitude into the phase axis before and behind the inversion pulse, acquiring second phase data similarly except for the inserting step, and determining the amount of a phase shift due to the influence of the reset pulse, based on the difference between the first phase data and the second phase data.
As described in the first to third aspects, the residual magnetization prior to the application of the phase encoding pulse can be always kept constant by using the reset pulse. However, the influence produced due to the insertion of the reset pulse must also be taken into consideration (eddy currents are considered as a typical one of the influence).
In the phase shift measuring method according to the fourth aspect, the amount of the phase shift due to the influence of the reset pulse can be determined from the difference between the phase data at the non-insertion of the reset pule and the phase data at the insertion of the reset pulse.
In a fifth aspect, the present invention provides an MR imaging method according to the third aspect, wherein the area of the reset pulse or compensating pulse is changed so as to compensate for the amount of the phase shift measured by the phase shift measuring method according to the fourth aspect.
In the MR imaging method according to the fifth aspect, an FSE sequence, which has corrected the phase shift due to the influence of the reset pulse, can be executed.
In a sixth aspect, the present invention provides an MR imaging system comprising RF pulse transmitting means, gradient pulse applying means, NMR signal receiving means, the MR imaging system executing MR imaging by a high-speed spin echo process for controlling the respective means to thereby transmit an inversion pulse after the transmission of an excitation pulse, apply a phase encoding pulse to a phase axis, collect data from echoes while a read pulse is being applied to a read axis, repeat the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse and collect data from a plurality of echoes by one excitation, and reset pulse applying means for inserting a positive or negative reset pulse having amplitude equal to the maximum amplitude used in an arbitrary gradient axis or having greater amplitude into the arbitrary gradient axis before and behind the inversion pulse.
The MR imaging system according to the sixth aspect is capable of suitably executing the MR imaging method according to the first aspect.
According to the MR imaging method of the present invention as described above, the influence of the variation in residual magnetization, which is caused by the change in gradient magnetic field pulse, can be prevented.
According to the phase shift measuring method of the present invention as well, the phase shift in subsequent echo due to the influence of the eddy currents and residual magnetization caused by the preceding phase encoding pulse or the like in the FSE sequence can be measured by matching with the actual FSE sequence.
Further, according to the MR imaging system of the present invention, the above-described method can suitably be implemented.
Further objects and advantages of the present invention will be apparent form the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.